Automated fiber placement (AFP) is an important manufacturing process in composite aerospace part manufacturing and has attracted much interest since future aircraft programs, such as the Boeing 787 and Airbus A350XWB, contain more than 50% by weight of advanced composite components. Also, the use of robot manipulators increases the flexibility of the fiber placement process and allows for the fabrication of more complex structures. Existing AFP research discusses productivity, steering and control, processing conditions, materials, layup modeling and simulation, and functional integration. Robotics work mainly focuses on path planning for AFP while typically a point-cloud is generated for the AFP head to follow to lay the material onto the mold.
Although many robot based AFP systems have been proposed and studied, this technology is still not widely used in industry where manual lay-up is still the main method due to cost constraints and level of complexity of molds. Furthermore, all previous work used serial robots as operation arms to hold the fiber placement head due to the fact that they are widely developed and used in automatic industry. However, serial robots generally have low stiffness and large inertia due to their serially connected structure, which affects their force and precision performance in high-compact-force applications, like AFP. In contrast to serial robots, parallel robots have multiple support limbs with low inertia, high structure stiffness, good positioning accuracy and high speeds. Based on this, they are widely used in the industrial applications requiring high speed and stiffness. Thus in this paper for the first time, a parallel mechanism is introduced in AFP and an optimal design is proposed as a basis for AFP.
In general a 6DOF platform is needed for the AFP operation to have flexibility in manufacturing all kinds of parts with complex mold surfaces. Considering the need of a moving platform to support the parallel mechanism and spindle rotation of the placement head on the parallel mechanism, a 4DOF parallel mechanism with 2T2R (two translations and two rotations) will be sufficient for automated fiber placement. In parallel mechanism research, 6DOF ones have been studied extensively with the Stewart-Gough platform with later focus moving to parallel mechanisms with less than 6DOF represented by many 3DOF ones. Due to the complexity of coupling between rotation and translation and singularity issues, 4DOF parallel mechanisms have not been investigated much and related work is mainly on synthesis. A class of asymmetrical 2T2R parallel mechanisms was synthesized in while symmetrical ones were obtained in using screw theory. Focusing on two rotation motions, and synthesized new 2T2R parallel mechanisms using general function set and Lie group theory respectively. Considering the requirement of a moving base (one translation) and a rotating spindle (one rotation), 2T2R parallel platforms have been used in 5- or 6-axis machine tools. A 2PRR-2PUS parallel mechanism with a moving platform formed by two parts joined with a revolute joint was proposed for a 5-axis machine tool. Reachable workspace of a 2PSS-2PUS parallel mechanism with two spherical joints coincided was studied in for machine tool applications while four possible singularity configurations of a 2UPR-2UPS parallel mechanism were obtained in. Recently, for needle manipulation tasks a class of 2T2R parallel mechanisms were synthesized using screw theory. However, little work has been found on singularity-free workspace analysis and optimal design of 2T2R parallel mechanisms.